What the gorse spider mite does to gorse
Tetranychus lintearius lives in communal webs on gorse. Its sucking mouthparts pierce the walls of single foliage cells and extract the contents, so the foliage looks bleached or even brown, growth is reduced and flowers may abort. The mite damages gorse only.
The damage depends on how long the attack lasts. If the mites stay on a bush for one year, it is stunted that year but recovers and grows normally the next. If large numbers stay for several years, the damage is more permanent: in one study, bushes attacked for two successive years were on average 20% lighter than undamaged bushes.
Colonies grow for two to three years and then crash suddenly in midsummer. Predators eat the less mobile juveniles and the adults migrate in a mass; they gather on branch tips, can drip from them like icicles, and are blown by wind to new bushes. Satellite colonies appear about a year later, often some distance away, and spread fastest in the direction of the prevailing wind.
How to recognise the mite and its lookalikes
The mites live in fine white communal webs that may be as small as a fist or cover several bushes. They are easiest to find after warm dry spells, when colonies are largest, and rain damages the webbing. In cold weather they cluster in the centre of the web, often on the leeward side of stems. Adults are brick red, females are smaller than a pinhead, and there are about five females for every male. Each female lays 30 to 40 eggs, usually one a day. With a magnifying glass, look for clusters of tiny round brownish eggs tucked close to the stems, and do not confuse them with the much smaller black specks of frass (droppings). New adults feed for a fortnight before breeding, and the warmer it is the faster the mites develop. Eggs hatch after about a fortnight and the young pass through six juvenile stages, leaving cast-off white skins in the webbing.
Cultural and physical alternatives
Weeds in organic horticulture cannot be controlled by any chemical method, so the options are false and stale seedbeds (grow the weeds, kill them by thermal or mechanical cultivation, then sow the crop), rotating weed-susceptible crops such as onions with weed-suppressant crops such as potatoes, plastic mulches or weed-control fabric, composted manure (composting kills the weed seeds in it), transplants, which put the crop a step ahead of the weeds, green manure, which helps suppress weeds, and equipment such as an oscillating hoe or a tractor-mounted brush weeder. The aim is to keep weeds managed, not to eradicate them. For cover crops, see the green manure guide.
For pests in Irish organic horticulture, use natural vegetation around fields to encourage natural predators, fleece or bio-nets as insect barriers, and an understory under brassicas to discourage caterpillar egg-laying.
Grass weeds on Irish tillage land
For wild oats, bromes, blackgrass and canary grass, cultural and hygiene measures come first. A shallow stale seedbed to 5 cm takes one flush of weeds. Harvest and bale fields with grass-weed problems last, and clean the combine and baler thoroughly when entering or leaving badly infested fields. Machinery bought new or brought in by a contractor must be cleaned before it enters the farm, and the origin of straw in imported farmyard manure needs checking. Source certified seed for main crops and cover crops whenever possible. Capture a ripe seed sample and test it for herbicide resistance. In the crop, hand-rogue, or mow heavily infested fields for silage early, before the weeds shed seed. In the rotation, change species between autumn and spring to allow a change of herbicide mode of action, delay autumn planting, raise the seed rate by 15 to 20% or sow a short-term ley of three to five years; the table rates each option differently for each weed.
Keep your spray records in one place. Free to start.
Trials with organisms in Ireland
Proposals to trial micro-organisms, macro-organisms or adjuvants go through the APPPT form of the DAFM trials route. For a trial that releases a non-native micro-organism, approval is first required from the National Parks and Wildlife Service (NPWS), before DAFM trial approval.
Before applying, give DAFM and NPWS the scientific name of the organism, where it comes from, who supplies it, how much you want, where you would use it and why, and say whether it would stay contained or reach the open environment. Get their answer in writing before you order, transport or release anything.
Ragwort flea beetle: what it does to ragwort
The ragwort flea beetle (Longitarsus jacobaeae) is an insect used against ragwort in other countries. Its larvae feed within the roots and leaf petioles of ragwort plants during autumn, winter and spring. In a field experiment in Oregon, flea beetles alone reduced vegetative ragwort densities by 95% and flower production by 39% compared with plants in control plots. A farmer cannot simply order the beetle: for an organism released in Ireland, follow the approval route in the section above before you order, transport or release anything. For what you can do on your own grassland, see the grassland spray records.
Before you accept a biological weed killer or product
Identify the weed first. Photograph the whole plant and its flowers or seed heads, note the crop or land use and the location, and mark whether it is a few plants, a dense patch or a field-wide problem. Do not order an organism or product while the weed's identity is still a guess; record the adviser's identification against the marked area.
Decide what you need: stopping seed return, protecting crop establishment, keeping access open or shrinking a perennial patch. A method that solves one of these can leave another untouched.
Ask the seller for the exact organism or substance, the formulation, the target weed and the trial report behind the claim. Ask what the trial measured (leaf damage, less seed, fewer weeds in later crops), how long it ran, where, and what the comparison was. A laboratory feeding test is not an outdoor establishment trial, and a photo of webbing, a count of living mites and the weight of a gorse bush are three different measurements. Ask for the full publication behind a headline percentage and check the sample size and whether the result was repeated. A lighter bush is not proof of eradication. If the product is sold as a pesticide, look up its exact identity through the Irish product-register route.
Plan the follow-up
Before the work, mark and photograph the patch, then photograph the same view later, with dates. Write down other things that change the picture: cutting, grazing, cultivation, flooding, storm damage or a new crop. If the weed declines, check its identification, the weather and the other organisms present before ordering another consignment. If a herbicide has failed, gather the treatment and field history first (see the Italian ryegrass guide); surviving weeds alone do not show that you need a biological alternative. Agree who will check regrowth and when to ask for advice again.
Next step
Related: the cover-crop guide and the Italian ryegrass guide.